Catching uncaught exceptions in Python

Now that I have my blog set up and it looks great, I’d like to tell you all about how I was introduced to post_mortem.

I’ve been slowly working on and off on a CLI tool to query DNS records because of an issue I found at work. Initially I didn’t realize that this had already been done until one of my friends had told me:

“Oh, you’re implementing dig? Nice!”

Anyways, I had been using the dnspython library to do this, but I was being very specific with my imports, which meant the exception classes I needed were not being imported. For example, in dnspython there is the NoAnswer exception class, which gets thrown when the DNS response does not contain an answer to the question, such as:

“What are the TXT records of a given domain?” when the domain doesn’t have TXT records.

I could not catch this exception until I explicitly imported it, but there were even more situations that I could not catch because I just didn’t know about them!

So I asked a friend for help, and they told me to try pdb. I don’t use pdb often, but I did this time and was still stuck, then my friend said to try post_mortem, and to wrap it around the main function block.

I included here so you can see how easy it is:

if __name__ == '__main__':
    try:
        main()
    except Exception as e:
        from pdb import post_mortem
        post_mortem()

It is pretty simple, but it allowed me to climb up and down the stack and find out what exceptions weren’t being caught so that I could explicitly import their respective classes. It really blew my mind to have such a powerful tool, it must be new, right? Nope! It’s been in Python since Python 2.

Thanks for reading!